Full Adder Question

Does "255" require eight full adders in a digital circuit? I want to make sure I understand this correctly: A full adder outputs a sum 'plus' a carry bit. The sum could be fed directly to an LED but the carry bit is fed to the next full adder. If I built this circuit is this correct?
Larry

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Comments

  • Here is a tutorial on adders.
    Re-inventing the wheel is not a waste of time if, when you are done, you understand why it is round.
  • lardomlardom Posts: 1,532
    edited 2019-01-24 - 20:48:44
    @tomcrawford, thanks. I wanted to be sure I understood what I was looking at. The tutorial had a diagram that makes it clear. "255" would require seven full adders plus an XOR gate for the MSB.
    (edit: the LSB only needs a half adder)
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    Larry

    If the grass is greener on the other side...it's time to water your lawn.
  • If you look at the 74LS181 data sheet you will find the logic equivalent circuit. This implements a carry forward circuit that is much faster. I don’t think any of these 74xx181 are available any more.
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  • Cluso99 wrote: »
    If you look at the 74LS181 data sheet you will find the logic equivalent circuit. This implements a carry forward circuit that is much faster. I don’t think any of these 74xx181 are available any more.

    There are a few still available. Jameco is sitting on 175. They do not look like pulls.

    https://www.jameco.com/z/74LS181-Major-Brands-4-Bit-Arithmetic-Logic-Unit-DIP-24_46973.html
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  • 74xx181 is EOL'd
    74HC283 may be its closest living relative (4 bit full adder)

    If you want to emulate a 74xx181 I think a rom lookup table would be the way to go as its stateless as
    far as I can tell.
  • If you google "74181", you'll find several tutorials on the '181, including its very own wikipedia page.

    As to using a rom lookup table, there was once a machine that used a lookup table to do addition. Decimal, I think.

    It was a "CADET", which stood for "Can't Add, Doesn't Even Try".
    Re-inventing the wheel is not a waste of time if, when you are done, you understand why it is round.
  • How do I add new topic?
  • Iguanaman wrote: »
    How do I add new topic?

    Answered in a Private Message.
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